The present invention generally relates to an optical image projecting system and, more particularly, to an image projecting system utilizable in a combined microfilm reader-printer apparatus of a type wherein the printer unit employs a slit-exposure type electrophotographic copying machine.
In the conventional combined reader-printer apparatus of a type employing the optical image projecting system wherein the distance of the optical path from the support plane, at which an original is placed, to the image plane where a photoreceptor member forming a part of the electrophotographic copying machine is located for receiving the image of the original projected thereonto by means of a projecting lens system, is fixed and which has a capability of magnifying the image of the original to a desired scale relative to the size of the original image, it is well known to those skilled in the art that adjustment of the magnification of the original image to be, or being, projected onto the image plane and, hence, the photoreceptor member is achieved by adjusting the conjugate distance, replacing the projecting lens system of a given focal length with another projecting lens system of different focal length, or varying the speed ratio of the speed of movement of the original along the support plane relative to the speed of movement of the photoreceptor member along the image plane.
However, in view of the fact that the optical system used in the conventional combined reader-printer apparatus is required to have a relatively large magnifying capability so that an image recorded on a microfilm on a reduced scale can be enlarged and, also, in view of the fact that the speed of movement of the original, that is, the microfilm, is generally low as compared with the speed of movement of the photoreceptor member, a difficulty has heretofore been involved in achieving a synchronization between the speeds of movement of the microfilm and the photoreceptor member. In other words, a difficulty has heretofore been involved in accurately determining the particular speed ratio each time the magnification is changed.
In addition, in view of the fact that scales of the same image recorded on respective microfilms often vary from one microfilm to another, the reader-printer apparatus is required to have a complicated and expensive transmission system effective to change the above described speed ratio where the magnification adjustment is desired to be performed by changing such speed ratio, replacing one lens system with another or adjusting the conjugate distance.
This will be discussed with reference to FIG. 1 of the accompanying drawings which illustrate the principle of the optical system used in the conventional reader-printer apparatus. Referring to FIG. 1, the photoreceptor member is shown in the form of a photoreceptor drum 5, which drum 5 is adapted to be rotated along the image plane 4 in the direction shown by the arrow at a predetermined peripheral velocity V. The image of the original 2, that is, the image on the microfilm, is enlarged by a projecting lens assembly 1 and then projected onto the photoreceptor drum 5 through a slit 6 to form an electrostatic latent image thereon. Assuming that the magnification of the image of the original being enlarged is X, the microfilm must be moved at a velocity of V/X along the support plane in a direction counter to the direction of rotation of the photoreceptor drum 5.
On the other hand, in view of the fact that the size of the image on each frame of the microfilm varies from one microfilm to another according to the difference in microfilm standards, adjustment of the magnification is required in order for all of the images recorded on different microfilms to be projected on the same scale. Accordingly, if the magnification factor X is subsequently changed to a different magnification factor X', not only must the speed of movement of the microfilm be adjusted to V/X', but also either the projecting lens assembly 1 if the projecting lens system is so constituted must be replaced with a different projecting lens assembly 1' or the focal length of a zoom lens if the projecting lens system is so constituted must be adjusted.
In view of the above, while the size of each frame of different microfilms varies from maximum to minimum over a relatively large range, to make the speed of movement of the microfilm to cope with a magnification factor selected in consideration of the range over which the size of each frame varies from one microfilm to another cannot be achieved by a simple speed adjusting means.